Solid state relay

Discussion in 'General Electronics Chat' started by rubicon99, Apr 30, 2012.

  1. rubicon99

    Thread Starter Member

    Apr 19, 2012
    46
    0
    What is the difference between a solid state relay and an electro mechanical relay other than that one uses mosfets.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Actually they mostly use Triacs, for AC. DC models use MOSFETs.

    You have to understand using a coil as an electromagnet to suck down a contact switch is just not energy efficient. Most SSR use an LED, which provide really excellent isolation, there is no electrical contact between the AC side and the DC side.

    The currents used to light the LED are low, and they have a wide voltage range, whereas a coil has a narrow range of voltages.

    This answer your question?
     
  3. rubicon99

    Thread Starter Member

    Apr 19, 2012
    46
    0
    Is there a voltage drop?
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    The DC input side of the relay has a range of voltage, usually written write on the case. Typically it is 3VDC to 7VDC at 3ma. 3ma is worst case, and they will not drag down the voltage unless it is part of a voltage divider. There is no drop.

    I am a fan of SSR's, they are much easier to use that relays.

    Safer too. The isolation I talked about will allow the printer port of an old computer to drive the SSR, which in turn can handle AC current.

    Their only disadvantage is they don't handle inductive loads well, read that as motors. They will still do the job if overrated though.
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    All the ones I've seen have been 3-32VDC. Probably because most everything I work with has a 24VDC control circuit.
     
  6. rubicon99

    Thread Starter Member

    Apr 19, 2012
    46
    0
    So I can use something like this to turn off and on a device just by giving a ttl signal from a microcontroller?
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Yes, they are ideal for this application.

    Basically they are a single on/off switch though.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    make sure you get a mosfet SSR if you're going to power a DC load though. If you get a triac SSR and try to use it on a DC load, it will turn on and never turn back off. In my experience, if it doesn't say mosfet on it, then it's a triac.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The output of an AC SSR has a drop of a volt or so from the SCR switch.

    The output of a DC SSR looks like a resistance whose value depends upon the current rating of the device. That value should be on the spec sheet.

    Mechanical relays have a very low resistance, of course, mainly due to the metal contacts.
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Thanks, I misunderstood the questions. The AC units are a lot less than a volt though, I've operated them at 5A, and there would have been more heat otherwise.
     
  11. rubicon99

    Thread Starter Member

    Apr 19, 2012
    46
    0
  12. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,576
    231
    Yes...

    Ken
     
  13. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Yes, but note they are limited to 2A maximum and will have a voltage drop of about 0.5V at that current.
     
  14. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    If you look at the data sheets for AC SSR's you will see that they have well over a volt forward drop. That's intrinsic to the SCRs or TRIACs typically used in AC SSRs.

    Perhaps you didn't notice the 7-8W dissipation because of the large package of the SSR.
     
  15. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    While it is possible, I have used up to 20A units on ovens. 20W (1V drop) is a lot of power.... I think I would have noticed.

    There is always a danger in generalizing, I'm generalizing one way and you the other. There is a large selection of units out there.

    I have also used them on controllers that were in sealed boxes with no heat sinking. If there had been any heat (and 1W does add up) it would have been painfully obvious. As is, these controllers (which I helped design) were a major success, and there were no meltdowns.

    Part of my machine maintenance days.
     
  16. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    I never really looked at the datasheets for the SSRs. I would be surprised if they dissipated a lot of heat, because usually no great care is taken to ensure they're properly heat sinked. They just get bolted to the back of a panel. i've only seen one die from heat, and it was operating at it's max, bolted to the panel.
     
  17. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Here's a datasheet for a 10, 25A, and 40A SS AC relay. It shows a 1.6V voltage drop for all of them. I believe that's quite typical of most SS AC relays.

    Edit: If you can show be one with a significantly lower drop, please do. ;)
     
  18. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    I cannot. I googled the 3 in my desk drawer:
    Crydom A2450 1.7vpk
    Crydom HA4890 1.6Vpk
    Magnecraft 6690axxszs-dc3 1.6Vpk

    Quite surprising. Electrical enclosure backplanes must be much better heat sinks than I give them credit for.
     
  19. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,576
    231
    rubicon has 3 threads going on this project...all related. He was looking for a DIP DC SSR.

    Ken
     
  20. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    A large metal panel, even if it's steel, can be a fairly good heat sink. if a 20A unit was just free air mounted, I'm sure it would get rather toasty.
     
Loading...